1. Field of the Invention
The invention relates to high frequency communications and in particular to high data rates and parallel multichannel broadcasting.
2. Discussion of Prior Art
In a known multichannel hf broadcast system four parallel 75 baud telegraphy channels are operated. This system employs multichannel FEK (Frequency Exchange Key) modulation in a standard 3 kHz channel bandwidth. This system, however, has limited capacity and in order to increase capacity various proposals include increase of number of channels and an increase in transmitter power to 50 KW since it has been found that reception range falls off rapidly when the number of channels exceeds two or three. Satisfactory high speed serial modems have not yet been produced, however, use of these would still require transmitter power of at least 16 KW.
Whereas a single FEK channel signal is very efficient as an rf power source (in that the radio transmitter need only have the same peak power rating as the required mean power) a multichannel signal requires a radio transmitter which has a very high peak to mean power rating. For example, to radiate the same mean power per channel as a signal 1 kW FEK channel, an eight channel system will ideally require a transmitter having a means power of 8 kW but a peak rating of 64 kW. Signal conditioning can help to reduce this power ratio and therefore the peak power rating, by limiting the peak-to-mean ratio of the multichannel signal before it is transmitted. With special signal processing this ratio can be reduced to about 4 to 1, but this will still require a transmitter of 32 kW to transmit a mean signal power of only 8 kW. Without signal conditioning the radio transmitters will either produce severe harmonic and intermodulation interference, or at worse be damaged by peak power transients.
Serial data signals will not suffer from this particular problem because the peak-to-mean ratio is only about 3 dB (for PSK modulation). Nevertheless, the performance of present day serial modems will not be as good as multichannel FEK because long range multipath radio distortion and interference will produce considerable symbol distortion. Self-adaptive equalising serial modems will be able to overcome this problem in future, but a substantial signal-to-noise ratio (typically 18 dB) will be required to operate these systems properly to achieve an acceptable output error rate at the data rates required.
Serial modems also suffer from the disadvantage that any interference, including narrow-band carrier signals, will produce severe errors in all the data channels. This does not happen with multichannel FEK.
Long range radio communications can only be achieved using sky-wave propagation. This will support many propagating modes with time dispersions lasting several milliseconds. This produces severe inter-symbol interference and frequency selective fading across the band-width which hampers hf communication.
FEK modulation can sometimes operate in these conditions because the transmitted symbol is normally much longer (13 ms at 75 bauds) than the dispersion of the channel and the two FEK frequency tones can provide a degree of frequency diversity.
There will, however, be many occasions when this is insufficient to overcome prevailing multipath conditions and the output error rate becomes very poor despite having a very high signal-to-noise ratio.
In communications it is very important to take into account not only the peak-to-mean power ratio problem, but also the multipath distortion produced by HF sky-wave signals. This dispersion can be as high as 10 ms but it will be typically 8 ms and much less for nearly all but the very worst circuits.
Spread spectrum signals are very effective in counteracting multipath distortion because cross-correlation matched filter detection can be used to despread the received signal and simultaneously resolve the separate multipath components. Inter-symbol interference can therefore be avoided when spreading codes are used if the block length (the symbol period) is made longer than the total dispersal time for the signal path.
Inband interference, however, can obliterate an otherwise good data signal in any communications system. The inventor has found that interference in the hf band is generally narrow-band and usually less than 200 Hz.
The object of the invention is to provide a multichannel hf broadcast system with improved performance compared with the known systems.
The invention provides:
a multichannel high frequency (hf) communications system comprising a spread spectrum signal transmitter including:
a) means to encode a message signal by selection of at least one of a preselected number of semi-orthogonal binary codes;
b) means to modulate a hf carrier wave with the encoded signal; and
c) means to transmit the modulated hf signal;
and a receiver including:
a) means to detect the received modulated hf signal;
b) means to Fourier transform the received signal using a fast Fourier Transform (FFT) processor;
c) means to test for an excise any narrow frequency band interference; and
d) cross-correlation matched filter detection means to decode the Fourier transformed signal to derive the original message signal.
Pseudo-orthogonal codes are well known in the art, having a high impulsive autocorrelation function and low cross correlation function. Since the spectrum modulation results in the frequency spread of the wanted signal being greater than the frequency spread of typical interference, it is possible to excise interference to improve the communications.
The interference excision means preferably comprises means to compare signal frequency components symmetrically placed with respect to the carrier frequency and to excise any such component whose amplitude exceeds the amplitude of the respective symmetrical component by more than a predetermined limit.
Alternatively the interference excision means comprises means to compare signal frequency components symmetrically placed with respect to the carrier frequency and to assign a low weight to any such component whose amplitude exceeds the amplitude of the respective symmetrical component by more than a predetermined limit.
The threshold for interference excision can be made adjustable to maximise system performance. Phase information on symmetrically compared signal frequency components can also be used for interference excision.
In an advantageous arrangement the transmitter includes a data multiplexer capable of multiplexing a number of parallel data channels, the multiplexer being connected to the input of the encoding means. Advantageously channel synchronisation, data control and checking information is added to the multiplexed data connected to the encoding means.
In a particular form of the invention the coded data is low pass filtered and then connected to a PSK modulator to modulate a sub-carrier signal of the same frequency as the output bit rate from the encoding means. Preferably the low pass filter is an impulsive response Kaiser Bessel filter.
In the preferred arrangement the receiver includes means to correct the received signals for Doppler frequency shift and to detect the sub-carrier signal. The matched filter detector preferably comprises synchronous detectors to which the sub-carrier signal is respectively connected in phase quadrature. Advantageously the synchronously detected signal is connected in parallel to a plurality of cross-correlation detectors, each cross-correlation detector having stored therein a respective one of the preselected number of Fourier transformed pseudo-orthogonal codes.
Preferably the outputs of each cross-correlation detector is connected to a comparator circuit which identifies and decodes the most likely detected code by measurement of the largest cross-correlator output signal.